1. Field of the Technology
The present technology relates to an optical pickup apparatus which is installed in information processing equipment for performing at least one of information recording process, information reproducing process, information erasing process, and information rewriting process on an optical recording medium, and the optical pickup apparatus acts to emit light to the optical recording medium as well as to receive light reflected from the optical recording medium.
2. Description of the Related Art
An optical pickup apparatus is designed to effect emission and reception of light for an optical recording medium such for example as Compact Disk (CD for short), Digital Versatile Disk (DVD for short), and Btu-ray Disk (BD for short). The optical pickup apparatus is installed in information processing equipment for performing at least one of information recording process, information reproducing process, information erasing process, and information rewriting process on the optical recording medium. A semiconductor laser element is mounted in the optical pickup apparatus to effect emission and reception of light for use in information processing operation to be performed on the optical recording medium.
For optical disks of so-called CD-type, such as CD and recordable CD, adaptable to at least one of information recording process, information reproducing process, information erasing process, and information rewriting process that are performed by information processing equipment, a laser light source for emitting light having an emission wavelength close to 780 nm is used to execute signal processing. On the other hand, for optical disks of so-called DVD-type such as DVD and recordable DVD, a laser light source for emitting light having an emission wavelength close to 650 nm is used to execute signal processing. In addition, for BD, a laser light source for emitting light having a wavelength close to 405 nm is used to execute signal processing.
There has recently been an increasing demand for an optical pickup apparatus capable of performing processing operation on optical recording media of different types with adaptability to several wavelengths. However, as compared with an optical pickup apparatus designed to perform processing operation on an optical recording medium of a specific type, the one having the aforestated capability is made complicated in structure and grows in size. As a related art devised to overcome this problem, there is a known technology to implement an optical system common to focus error detection and radial error detection refer to Japanese Unexamined Patent Publication JP-A 11-73658 (1999), for example).
FIGS. 13A and 13B are plan views showing a light-receiving element and a diffraction element 1 of an optical pickup apparatus pursuant to the related art as viewed in a direction of optical axis Z. Specifically, FIG. 13A is a plan view of the light-receiving element, and FIG. 13B is a plan view of the diffraction element as viewed in the optical axis direction Z. The diffraction element 1 acts to diffract backward light, which is light reflected back from an optical recording medium, to direct ± first order diffraction light toward the light-receiving element. The diffraction element 1 has a circular outer shape, and is placed with its center O located at a position passing through the optical axis of the optical pickup apparatus. Moreover, the diffraction element 1 is divided into six regions by a first dividing line 3 extending in a tangential direction while passing through the center O and a second dividing line 4 and a third dividing line 5 that are each perpendicular to the first dividing line 3. On the other hand, the light-receiving element is divided into first to eighth regions R1 to R8, of which the second region R2, the fourth region R4, the sixth region R6, and the eighth region RB are each subdivided into three small regions a, b, and c.
In FIG. 13B, a right-left direction as viewed in the paper sheet with FIG. 13B printed thereon will be defined as the tangential direction, and a top-bottom direction as viewed in the paper sheet will be defined as a radial direction, which is a direction radially of the circular plate-shaped optical recording medium. The radial direction is perpendicular to the tangential direction. In a strict sense the radial direction comprises two opposite directions, namely one radial direction which is an upward direction as viewed in the paper sheet and the other radial direction opposite from one radial direction. Out of the second and third dividing lines 4 and 5 extending in parallel with the radial direction, the one located on the one-tracking-direction side (of the diffraction element) is the second dividing line 4.
Out of the six separate regions of the diffraction element 1 shown in FIG. 13B, the one radial direction-side region located at extreme one-tangential-direction position and the one radial direction-side region located at extreme the other-tangential-direction position correspond to, of the eight regions of the light-receiving element, the first region R1 and the fifth region R5. A beam of light which enters these regions is condensed so as to be focused on a light-receiving surface of the light-receiving element.
Out of the six separate regions of the diffraction element 1 shown in FIG. 13B, the other radial direction-side region located at extreme one-tangential-direction position and the other radial direction-side region located at extreme the other-tangential-direction position correspond to, of the eight regions of the light-receiving element, the third region R3 and the seventh region R7. A beam of light which enters these regions is condensed so as to be focused on the light-receiving surface of the light-receiving element.
Out of the six separate regions of the diffraction element 1 shown in FIG. 13B, the one radial direction-side region located centrally in the tangential direction corresponds to, of the eight regions of the light-receiving element, the second region R2 and the eighth region R8. A beam of light which enters these regions is condensed so as to be focused farther than the light-receiving surface of the light-receiving element from the diffraction element 1.
Out of the six separate regions of the diffraction element 1 shown in FIG. 13B, the other radial direction-side region located centrally in the tangential direction corresponds to, of the eight regions of the light-receiving element, the fourth region R4 and the sixth region R6. A beam of light which enters these regions is condensed so as to be focused closer than the light-receiving surface of the light-receiving element to the diffraction element 1. As employed herein, “correspond to” means that reflected backward light diffracted in each of the regions of the diffraction element 1 enters its respective region of the light-receiving element.
If forward light entering the optical recording medium travels radially across a track of the optical recording medium, a significant luminance variation will result in two areas located tangentially centrally but spaced apart radially. In FIG. 13B, such an area which exhibits a significant luminance variation is encircled by a chain double-dashed line. In the diffraction element 1, in other areas than the area exhibiting a significant luminance variation, even if the forward light travels radially across the track, the degree of luminance variation is relatively low. By exploiting a difference in light quantity between a light beam entering the area exhibiting a significant luminance variation and a light beam entering the area exhibiting a slight luminance variation, it is possible to produce a push-pull signal. In response to the push-pull signal thus produced, tracking servomechanism (not represented graphically) is driven thereby to allow radial adjustment of the forward light which enters the optical recording medium.
In the optical pickup apparatus pursuant to the related art, a radial error for tracking servo control is obtained by the differential push pull method using the quantity of a light beam entering each of the regions of the light-receiving element. To be specific, the radial error is calculated in accordance with the following formula:
                                                                           DPP                =                                ⁢                                                      {                                                                  (                                                                              R                            ⁢                                                                                                                  ⁢                            4                            ⁢                            all                                                    +                                                      R                            ⁢                                                                                                                  ⁢                            6                            ⁢                                                                                                                  ⁢                            all                                                                          )                                            -                                              (                                                                              R                            ⁢                                                                                                                  ⁢                            2                            ⁢                            all                                                    +                                                      R                            ⁢                                                                                                                  ⁢                            8                            ⁢                                                                                                                  ⁢                            all                                                                          )                                                              }                                    -                                                                                                                        ⁢                                  k                  ×                                      {                                                                  (                                                                              R                            ⁢                                                                                                                  ⁢                            1                                                    +                                                      R                            ⁢                                                                                                                  ⁢                            5                                                                          )                                            -                                              (                                                                              R                            ⁢                                                                                                                  ⁢                            3                                                    +                                                      R                            ⁢                                                                                                                  ⁢                            7                                                                          )                                                              }                                                                                                                          =                                ⁢                                  MPP                  -                                      k                    ×                    SPP                                                                                                            (          1          )                    
In the formula (1), DPP represents a push-pull signal which is obtainable by the differential push pull method, and R1 to R8 represent the quantities of the light beams entering the first to eighth regions, respectively, of the light-receiving element. Out of the first to eighth regions, the second, fourth, sixth, and eighth regions are each subdivided into three small regions. In the formula (1), R2all, R4all, R6all, and R8all represent the quantities of the light beams entering the second region, the fourth region, the sixth region, and the eighth region, respectively, and in a sense, each value represents the sum total of quantities of the light beams entering the three small regions, respectively. Note that, specifically, the quantity of the light beam entering each of the regions of the light-receiving element is expressed as the signal strength of an electric signal outputted through conversion by the light-receiving element.
As shown in the formula (1), MPP is equivalent to (R4all+R6all)−(R2all+R8all). In this case, since MPP involves the quantity of the light beam entering the area exhibiting a significant luminance variation, when the forward light entering the optical recording medium travels across the track of the optical recording medium, MPP is caused to vary greatly so as to indicate a radial displacement, or an AC component of a radial error signal.
Moreover, SPP is equivalent to (R1+R5)−(R3+R7). In this case, since SPP does not involve the quantity of the light beam entering the area exhibiting a significant luminance variation, even if the forward light entering the optical recording medium travels across the track of the optical recording medium, SPP is smaller than MPP in the extent of variation in light quantity relative to the amount of radial displacement. Note that k represents a coefficient and, by multiplying SPP by this coefficient, it is possible to render the rate of change of the second term of the formula (1) relative to the amount of deviation of the objective-lens position from the track equal to the rate of change of MPP. On the basis of MPP, SPP, and k, a DC component of the push-pull signal can be produced thereby to reflect a radial error offset upon the push-pull signal. The three small regions obtained by subdividing each of the second, fourth, sixth, and eighth regions of the light-receiving element are used for focus error detection.
Out of the terms constituting the formula (1), (R4all+R6all) and (R1+R5) ideally stand in positive correlation with each other relative to tracking deviation, in other words, the terms are of the same polarity. Similarly, there is a positive correlation between (R2all+R8all) and (R3+R7). In a case where (R4all+R6all) and (R1+R5) representing the quantity of light entering each region of the light-receiving element; that is, the strength of a signal outputted from each region of the light-receiving element, are ideally increased and decreased in a correlative manner, a stable push-pull signal DPP can be produced as a servo signal for radial error correction.
However, if a lack of synchronization occurs between (R4all+R6all) and (R1+R5) that are fundamentally the same in polarity due to the presence of a foreign matter, a flaw of the optical recording medium, etc. with consequent mutual differences in signal fluctuation, the DPP signal will fluctuate in amplitude, which gives rise to the problem of a failure in stable radial servo control. The degree of deviation of the push-pull signal DPP resulting from a foreign matter, a flaw, or the like may possibly be greater than the radial error-induced variation of the push-pull signal DPP.
FIG. 14 is a plan view of the diffraction element 1 of the optical pickup apparatus of the related art as viewed in the optical axis direction Z, illustrating a shade 2 caused by a foreign matter adherent to the surface of the optical recording medium and how the shade 2 travels. In the presence of a foreign matter or flaw on the optical recording medium, the forward light cannot be reflected properly from the foreign matter- or flaw-bearing part of the optical recording medium, with the result that the quantity of the reflected backward light is decreased. As the optical recording medium is turned, the foreign matter moves tangentially in conjunction with the track relative to the optical pickup apparatus. In FIG. 14, there is shown a case where the foreign matter is circular in shape and has a diameter smaller than the distance between the second dividing line 4 and the third dividing line 5 of the diffraction element 1.
As the foreign-matter shade 2 comes near to the diffraction element 1 where the reflected backward light enters, it is located in, of the three tangentially separated regions, the one assuming extreme the other-tangential-direction position. At this time, the amount of light received by each of the first region R1, the third region R3, the fifth region R5, and the seventh region R7 of the light-receiving element is decreased. When the foreign-matter shade 2 is shifted radially from the position passing through the center of the diffraction element 1, there arises a difference between the sum total of the strengths of signals from the first region R1 and the fifth region R5 of the light-receiving element and the sum total of the strengths of signals from the third region R3 and the seventh region R7 of the light-receiving element. This condition of the optical pickup apparatus of the related art will be referred to as “the first condition”.
Next, when the foreign-matter shade 2 is located between the second dividing line 4 and the third dividing line 5 on the diffraction element 1, the amount of light received by each of the second region R2, the fourth region R4, the sixth region R6, and the eighth region R8 of the light-receiving element is decreased. When the foreign-matter shade 2 is shifted radially from the position passing through the center of the diffraction element 1, there arises a difference between the sum total of the strengths of signals from the second region R2 and the eighth region R8 of the light-receiving element and the sum total of the strengths of signals from the fourth region R4 and the sixth region R6 of the light-receiving element. This condition of the optical pickup apparatus of the related art will be referred to as “the second condition”. Further, when the foreign-matter shade 2 is located in the region assuming extreme one-tangential-direction position, the amount of light received by each of the first region 21, the third region R3, the fifth region R5, and the seventh region R7 of the light-receiving element is decreased. When the foreign-matter shade 2 is shifted radially from the position passing through the center of the diffraction element 1, there arises a difference between the sum total of the strengths of signals from the first region R1 and the fifth region R5 of the light-receiving element and the sum total of the strengths of signals from the third region R3 and the seventh region R7 of the light-receiving element. This condition of the optical pickup apparatus of the related art will be referred to as “the third condition”.
The first to third conditions are separated from one another time. By the timewise differences among the first to third conditions, the increase-decrease relationship between (R4all+R6all) and (R1+R5) of the formula (1) varies with time correspondingly. In consequence, they should ideally be of the same polarity, but cannot perfectly synchronized with each other in reality.
FIGS. 15A to 15C are diagrams showing the quantity of light detected in the light-receiving element when the foreign-matter shade 2 passes through a radially-shifted location on the diffraction element 1 of the optical pickup apparatus of the related art. FIGS. 15A to 15C are charts, showing states where the foreign-matter shade 2 is projected differently onto the diffraction element 1, on which the abscissa represents the tangential position of the foreign-matter shade 2 on the diffraction element 1 and the ordinate represents outputs produced from the light-receiving element. In FIG. 15A, the thick line indicates (R2all+R8all), and the thin line indicates (R1+R5). In FIG. 15B, the thick line indicates (R4all+R6all), and the thin line indicates (R3+R7).
FIG. 15C indicates the values of MPP, k×SPP, and DPP in the formula (1). If, in its tangential movement, the foreign matter travels in a location shifted from the radial center of the diffraction element 1, there arises a lack of synchronization in the outputs from the light-receiving element that should ideally be the same in polarity. As a result, as shown in FIG. 15C, there is a problem that DPP is caused to vary significantly.
A feature of an example embodiment presented herein is to provide an optical pickup apparatus that can be made compact and is capable of producing a stable push-pull signal.
According to the example embodiment, an optical pickup apparatus comprises a light source, an objective lens, a diffraction element, a light-receiving element, and a control-driving section. The objective lens is a lens for condensing light emitted from the light source on a surface of an optical recording medium. The light reflected from the optical recording medium enters the diffraction element. A light beam diffracted by the diffraction element enters the light-receiving element. The light-receiving element has a plurality of light-receiving regions. The light-receiving region produces an output signal responsive to the light quantity of the incident light beam. The control-driving section obtains differences among the output signals produced by a plurality of the light-receiving regions by calculation to derive a push-pull signal, and drives the objective lens under control on the basis of the push-pull signal.
The diffraction element has a forward region and a reverse region. The forward region serves to let a light beam enter, out of a plurality of the light-receiving regions, the one for producing an output signal bearing a same-sign relation to the push-pull signal. The reverse region serves to let a light beam enter, out of a plurality of the light-receiving regions, the one for producing an output signal bearing an opposite-sign relation to the push-pull signal. A plurality of the forward regions and the reverse regions are arranged alternately in one of the directions set for the diffraction element.
By virtue of alternate arrangement of a plurality of the forward regions and the reverse regions in one of the directions set for the diffraction element, in contrast to a case where a plurality of the forward regions and the reverse regions are not arranged in an alternating manner, it is possible to increase the possibility that a foreign-matter shade extends over the forward region and the reverse region adjacent to each other. Accordingly, in terms of the influence of the foreign-matter shade on the push-pull signal, the timewise difference between the output signal corresponding to the forward region and the output signal corresponding to the reverse region can be lessened. This makes it possible to achieve mutual cancellation of the influence of the foreign-matter shade on the forward region and that on the reverse region in the performance of push-pull signal calculation, and thereby suppress push-pull signal fluctuation resulting from the on-the-diffraction-element movement of the foreign-matter shade projected on the diffraction element. Moreover, radial error correction is carried out in accordance with push-pull signal calculation, wherefore the light-receiving element used to read signals recorded on the optical recording medium can be used also for control and driving of the objective lens. In consequence, as compared with a case where the light-receiving element is not adaptable to shared use, reduction in apparatus size can be achieved.
According to the example embodiment, the diffraction element has a bright-dark contrast area and a simple area. The bright-dark contrast area is an area where, of the light beams reflected from the optical recording medium, diffraction reflected light from a track borne on the surface of the optical recording medium enters. The simple area is an area where, of the light beams reflected from the optical recording medium, only simple reflected light from the optical recording medium enters; that is, no diffraction reflected light enters. The light-receiving region has a push-pull light-receiving region and an offset light-receiving region. Moreover, the diffraction element has a first diffraction region and a second diffraction region. The first diffraction region diffracts the incident light so that it can be directed to the push-pull light-receiving region. The second diffraction region diffracts the incident light so that it can be directed to the offset light-receiving region. A plurality of the first diffraction regions and the second diffraction regions are arranged alternately in one of the directions set for the diffraction element.
This makes it possible to decrease the possibility that the foreign-matter shade affects only one of the first diffraction region and the second diffraction region on the diffraction element, and thereby decrease the possibility that the foreign-matter shade affects only one of the push-pull light-receiving region and the offset light-receiving region of the light-receiving element. In this way, with the influence of the quantity of light entering the simple area taken away from the quantity of light entering the bright-dark contrast area on the diffraction element, at the time of push-pull signal calculation, it is possible to eliminate the influence of simple diffraction light entering the bright-dark contrast area, as well as to achieve cancellation of the influences of the foreign-matter shade. In consequence, a higher degree of accuracy in the push-pull signal is achievable compared to the related-art technology.
According to the example embodiment, in the diffraction element, a plurality of the forward regions and the reverse regions are arranged alternately in a tangential direction which corresponds to, out of the directions parallel to the diffraction element, the direction of a line tangent to the track at a position where light condensed by the objective lens enters the surface of the optical recording medium.
Thereby, when the shade of a foreign matter adherent to the surface of the optical recording medium moves in the tangential direction on the diffraction element, at least part of the foreign-matter shade passes through the forward region and the reverse region alternately several times. Accordingly, in terms of the influence of the foreign-matter shade on the push-pull signal, the timewise difference between the output signal corresponding to the forward region and the output signal corresponding to the reverse region can be lessened, wherefore push-pull signal fluctuation resulting from the on-the-diffraction-element movement of the foreign-matter shade projected on the diffraction element can be suppressed. Moreover, it is possible to decrease the possibility that the foreign-matter shade is projected lopsidedly on one side, i.e. one of the forward region and the reverse region. Therefore, the influence of the foreign-matter shade projected on the forward region and that of the foreign-matter shade projected on the reverse region cancel each other out with the consequence that push-pull signal fluctuation resulting from the movement of the foreign-matter shade on the diffraction element can be suppressed.
According to the example embodiment, a shape of the second diffraction region is so determined that a quantity of light entering the second diffraction region is proportional to an amount of deviation of the objective lens from the track.
In this case, in contrast to a case where the quantity of light entering the second diffraction region bears a non-linear relationship with the amount of deviation of the objective lens from the track, the formula used for push-pull signal calculation can be simplified, wherefore computation cost required for push-pull signal calculation can be reduced correspondingly. This makes it possible to obtain a radial error in a short period of time and with high accuracy, as well as to render the time taken for radial error correction as short as possible. Accordingly, the objective lens can be driven under control with higher accuracy.
According to the example embodiment, the diffraction element includes regions of which each constitutes at least part of either of the forward region and the reverse region, and at least part of the regions is rectangularly formed so that the tangential direction becomes its lengthwise direction.
This helps increase the possibility that, in the diffraction element, the foreign-matter shade extends over the forward region and the reverse region adjacent to each other in a radial direction. When the foreign-matter shade extends over the forward region and the reverse region adjacent to each other, in terms of the push-pull signal, the influence of the foreign-matter shade on the forward region and that of the foreign-matter shade on the reverse region can be synchronized with each other. Accordingly, push-pull signal fluctuation resulting from the on-the-diffraction-element movement of the foreign-matter shade projected on the diffraction element can be suppressed. This makes it possible to achieve mutual cancellation of the influence of the foreign-matter shade on the forward region and that on the reverse region.
According to the example embodiment, in the diffraction element, a plurality of the forward regions and the reverse regions are arranged alternately in a radial direction which is parallel to the diffraction element and perpendicular to the tangential direction.
By doing so, in contrast to the case of arranging a plurality of the forward regions and the reverse regions alternately in the tangential direction of the diffraction element, it is possible to easily insure timewise synchronization between the foreign-matter shade's projective entrance to the forward region and its projective entrance to the reverse region. Meanwhile, alternate arrangement of the forward regions and the reverse regions in the tangential direction affords the advantage to decrease the possibility that the foreign-matter shade is projected lopsidedly on one side, i.e. one of the forward region and the reverse region. By contrast, with the alternate arrangement of the forward regions and the reverse regions in the radial direction, the influence of the foreign-matter shade, which is projected so as to extend over the adjacent forward region and reverse region, on the forward region and that on the reverse region can be perfectly synchronized with each other. In consequence, the foreign-matter influence on the forward region and that on the reverse region cancel each other out.
According to the example embodiment, the diffraction regions constitute a first array and a second array. In the first array, a plurality of the forward regions and the reverse regions are arranged alternately in the tangential direction. In the second array, a plurality of the forward regions and the reverse regions are arranged alternately in the radial direction.
This makes it possible to form the first array in that part of the diffraction element in which a phase difference arises between the waveform of the output signal corresponding to the forward region and the waveform of the output signal corresponding to the reverse region, as well as to form the second array in that part of the diffraction element in which the waveform of the output signal corresponding to the forward region and the waveform of the output signal corresponding to the reverse region are in the same phase. Accordingly, in contrast to the case of arranging a plurality of the forward regions and the reverse regions alternately in only one specific direction in the diffraction element, in terms of the influence of the foreign-matter shade on the push-pull signal, the output signal corresponding to the forward region and the output signal corresponding to the reverse region can be synchronized with each other with a higher degree of accuracy. This helps stabilize the push-pull signal even further.
According to the example embodiment, the light-receiving element has an independent light-receiving region, and an output signal from the independent light-receiving region is not used for push-pull signal calculation. The diffraction element includes an independent diffraction region and the second diffraction region. The independent diffraction region is formed centrally of the diffraction element in the radial direction, and diffracts the incident light so that it can be directed to the independent light-receiving region. The second diffraction region is formed outwardly from the independent diffraction region in the radial direction.
By virtue of the independent diffraction region, the increase and decrease of the quantity of light entering the radial midportion of the diffraction element is independent of that of the push-pull signal. The second diffraction region is located radially outwardly from the independent diffraction region, the edges of which extends in the tangential and radial directions. Thereby, the quantity of light entering the radial midportion of the diffraction element that is relatively large in the intensity of incident light per unit area becomes irrelevant to the light quantity used for push-pull signal calculation. This leads to easiness in determining the shape of the second diffraction region in such a manner that the quantity of incident light is proportional to the amount of deviation of the objective lens from the track.
According to the example embodiment, the light-receiving region receives at least one of + first order diffraction light and − first order diffraction light resulting from diffraction in the diffraction element.
Accordingly, as compared with the case of utilizing zeroth order diffraction light in lieu of ± first order diffraction light, the distance between the diffraction element and the light-receiving element can be made shorter. Although there is a need to secure a sufficient distance between the diffraction element and the light-receiving element to space a plurality of the light-receiving regions apart in the light-receiving element for the sake of positive distinction, by utilizing ± first order diffraction light, it is possible to arrange a plurality of the light-receiving regions spacedly while shortening the distance between the diffraction element and the light-receiving element.
According to the example embodiment, the light-receiving element further includes a focus error detecting region, and the diffraction element further includes a focus-error diffraction region. The focus error detecting region included in the light-receiving element is a region for achieving focus error detection. The focus-error diffraction region included in the diffraction element acts to diffract incident light so that it can be directed to the focus error detecting region.
Thereby, focus error detection and radial error detection can be achieved by the common diffraction element and light-receiving element. Therefore, as compared with the case of disposing one of the diffraction element and the light-receiving element in a separate unit for focus error detection and radial error detection, the optical pickup apparatus can be made more compact.
According to the example embodiment, in response to an output signal from the focus error detecting region, the control-driving section produces a focus error signal by a knife edge method.
In this case, as compared with the case of effecting focus error detection by the differential push pull method, the light-receiving region used for focus error detection can be smaller in size, wherefore the optical pickup apparatus can be made more compact.
According to the example embodiment, in response to an output signal from the focus error detecting region, the control-driving section produces a focus error signal by a beam size method.
In this case, as compared with the case of effecting focus error detection by the differential push pull method for instance, the light-receiving region used for focus error detection can be smaller in size, wherefore the optical pickup apparatus can be made more compact.
According to the example embodiment, the light source, the diffraction element, and the light-receiving element are combined in a single-piece unit.
Accordingly, when mounted in combination with other components, the optical pickup apparatus can be handled with the light source, the diffraction element, and the light-receiving element secured in their relative positions. This leads to easiness in mounting operation.